1. TECHNICAL FIELD
The invention relates generally to circuits for powering fluorescent lamps. More specifically, the invention relates to ballast circuits for starting and operating fluorescent bulbs and lamps.
2. DISCUSSION OF RELATED ART
An important aspect of any fluorescent lighting system is a suitable ballasting technique. Over the years one of the most common ballast designs from reliability, compactness, and cost viewpoints has been an inductive ballast such as a laminated, iron-core inductive type. While these inductive ballasts are suitable in terms of electrical operating characteristics, they tend to incur substantial heat losses which result in a low efficiency. Furthermore, the inductive-type ballasts are quite heavy and this excessive weight often precludes having the ballast and lamp or bulb as an integral structure. For example, in a hand-held auxiliary lamp or emergency light, a standard inductive ballast is too heavy and cumbersome and hence is usually provided inconveniently as a separate element at the plug end of the power cord.
In addition to efficiency and weight considerations, ballast designs for fluorescent lights should provide for filament preheating prior to discharging the lamp to achieve more reliable starting. Filament preheating also extends the life of the lamp by preventing excessive material blow-off from the filaments upon discharge. Furthermore, continued but reduced filament heating during steady-state operation of the fluorescent light contributes to extended life.
Another important consideration is that DC operation can substantially shorten lamp life by a phenomenon called "mercury pump" which is the migration of mercury from one end of the lamp to the other. On low power and short-length bulbs this effect can be substantial. Also, high frequency operation results in improved lighting efficiency; though the effects of high frequency operation on lamp life are unknown, they are expected to be favorable.
Many attempts have been made to replace or improve the common inductive ballast with electronic circuits. Examples are shown in U.S. Pat. Nos. 4,320,325 issued to Anderson; 4,117,377 issued to Jimerson et al.; 4,378,514 to Collins; 4,463,286 to Justice; 4,388,562 to Josephson; 4,437,042 to Morais et al.; 4,484,109 to Buser and 4,330,736 to Perper. However, some of these devices still utilize a heavy inductive ballast and still others require a substantial number of components which inherently reduces reliability and maintainability while increasing costs. These negative factors are very undesirable since they obliterate any benefit of avoiding use of an inductive ballast.
It is apparent, therefore, that the need has long existed and continues to exist for a lightweight flourescent ballast which utilizes a reduced number of circuit elements so as to provide maximum reliability at minimal cost.